Integration of optical functions into photonic integrated circuits (PICs or “optical chips”) is a developing technology attracting increasing attention by researchers. In PICs optical functionality is created by bringing together basic optical components. One of these components is the polarization converter (or polarization rotator), which is used to control the state of polarization of light in a chip. This is important because the planar geometry typically results in polarization dependent operation, and because some applications are polarization based (e.g., polarization multiplexing in telecom applications). Polarization manipulation in PICs is also important for polarization independent operation of the integrated circuit chips, and for functionalities like polarization multiplexing and polarization switching. An ideal polarization converter would be a short low-loss passive component, which can be realized within the standard fabrication of a PIC. However, it still remains an unsolved challenge to realize such an ideal.
A number of proposals have been made for polarization converters. The most promising of these proposals seems to be the sloped sidewall devices, which operate as an integrated optical analogue of a half-wave plate. The tolerances to obtain an acceptable level of conversion, however, are relatively narrow. For example, the width deviations should be kept below 50 nm for conversion efficiency above 95%.
Thus, due to their tight fabrication tolerances, polarization converters known in the art are not adequate to make them commercially attractive.